The present invention relates to a video head drum for a video tape recorder.
Currently, video tape recorders of a helical scan type are most popular for business use, for instance in broadcasting stations. With respect to such video tape recorders, high image quality, high operational reliability and simple maintenance are required. Therefore, generally it is required that the vibrations of the rotary shaft of the rotary head drum of the video head drum be less than 5 .mu.m, and that the rotary head drum be replaceable by a new rotary head drum easily and speedily when its magnetic head or drum becomes abraded or damaged. Moreover, it is required that the rotary head drum be always replaceable with consistent accuracy of attachment of the replacement.
For that purpose, a video head drum as shown in FIG. 1 is conventionally known. In the figure, reference numeral 1 represents a lower guide drum; reference numeral 2, a rotary head drum; reference numeral 3, a rotary shaft; reference numeral 4, ball bearings; reference numeral 5, a flange portion of the rotary shaft 3; reference numeral 6, a nut; reference numeral 7, a magnetic head; reference numerals 8a and 8b, a rotary transformer.
The rotary shaft 3 is rotatably supported by a bearing portion 1a including the ball bearings 4, which bearing portion 1a is formed in the lower guide drum 1. The rotary head drum 2 is placed over an upper portion of the flange portion 5 of the rotary shaft 3 and supported by a sleeve 2a formed around a center hole of the rotary head drum 2. The rotary head drum 2 is fixed to the rotary shaft 3 by screwing a nut 6 on to an external thread portion 3a formed on the upper portion of the rotary shaft 3.
The magnetic head 7 is attached to a lower surface of the rotary head drum 2 in such a manner that the magnetic head 7 extends partly outward in the radius direction of the rotary head drum 2 from the gap formed between the lower guide drum 1 and the rotary head drum 2. Inputting signals to and outputting the same from the magnetic head 7 are performed through the rotary transformer 8a and 8b.
In the thus constructed video head drum, the rotary head drum 2 can be detached from the rotary shaft 3 by unscrewing the nut 6, and the rotary head drum 2 and the magnetic head 7 can be replaced speedily without any mechanical difficulty. In that case, if the end surfaces 2b and 2c of the sleeve 2a of the rotary head drum 2, the end surface 5a of the flange portion formed in the rotary shaft 3 and the end surface 6a of the nut 6 are at accurate right angles with respect to the axis of the rotary shaft 3, the rotary head drum 2 and the magnetic head 7 can also be attached with the required accuracy.
However, in view of the manufacturing techniques for those elements, guaranteeing the right-angle alignment of the end surfaces 2b, 2c, 5a and 6a with such accuracy will decrease the rate of production of the products, and will increase the production cost, to the extent that such accurate manufacturing is impractical. Therefore, when manufacturing those products, certain variations with respect to the accuracy of the right-angle alignments have to be allowed for in practice.
It may occur, therefore, that members without such perfect accuracy with respect to the right-angle alignment of the end surfaces 2b, 2c, 5a and 6a are employed. In such a case, in the conventional video head drum as shown in FIG. 1, it may occur that the rotary shaft 3 is misaligned or is bent, as shown with exaggeration in FIG. 1, when the rotary head drum 2 is fixed to the rotary shaft 3 with the nut 6, causing eccentric rotation of the rotary head drum 2 and vibrations of the rotary shaft 3. Once this takes place, the rotary head drum 2 cannot be used any longer in practice. Conventionally, in order to provide a solution to that problem, the clearance between the rotary shaft 3 and the sleeve 2a of the rotary head drum 2, when they are fitted together, is slightly excessive, and, when assembling, the correct alignment of the rotary head drum 2, where vibrations of the rotary shaft 3 can be minimized, is determined by gradually moving the rotary head drum 2 relative to the rotary shaft 3 within the clearance, while simultaneously rotating the rotary head drum 2, and employing a lever-type dial gauge indicator putting against the outer peripheral surface of the rotary head drum 2. When the correct position is found, the rotary head drum 2 is fixed to the rotary shaft 3 by fastening the nut 6.
This alignment adjustment procedure for keeping the shaft vibrations within 5 .mu.m requires some special measurement tools, such as the above-mentioned indicator, and special training for making such adjustment. Therefore, it is difficult for ordinary users of the video tape recorder, even the VTR operators in broadcasting stations, to replace the rotary head drum 2 and the magnetic head 7 by themselves, so that the cost for the maintenance of the video tape recorder is high. These are shortcomings of the conventional video tape recorders.